The present invention concerns a process for the manufacture of 3,6-dichloro-2-trichloromethylpyridine. More particularly, the present invention concerns a process for the manufacture of 3,6-dichloro-2-trichloromethylpyridine by vapor phase chlorination of 6-chloro-2-trichloromethylpyridine in the presence of a Type L zeolite catalyst.
3,6-Dichloro-2-trichloromethylpyridine (xcex1,3,6-penta) is a key intermediate for the production of the herbicide clopyralid, 3,6-dichloro-2-pyridinecarboxylic acid. However, xcex1,3,6-penta is difficult to obtain by direct chlorination. U.S. Pat. No. 3,420,833 describes the vapor phase chlorination of xcex1-picoline in which, among a mixture of chlorinated xcex1-picolines of varying degrees of chlorination, xcex1,3,6-penta and 4,6-dichloro-2-trichloromethylpyridine (xcex1,4,6-penta) arc produced in a ratio from 0.25 to 0.34. U.S. Pat. No. 4,713,460 discloses the vapor phase chlorination of 2,3-lutidine to provide a mixture of chlorinated picolines and lutidines containing upto 6.8 percent xcex1,3,6-penta U.S. Pat. No. 4,256,894 describes the liquid phase chlorination of 6-chloro-2-trichloromethyl-pyridine (xcex1,6-tet) in the presence of a Lewis acid type catalyst. While a mixture of products is typically obtained, at maximium concentrations of xcex1,3,6-penta, the ratio of xcex1,3,6-penta to 5,6-dichloro-2-trichloromethylpyridine (xcex1,5,6-penta) ranges from 0.18 to 0.53, more typically from 0.34 to 0.38. Even with optimization with respect to the level of xcex1,3,6-penta by removal of HCl as taught in U.S. Pat. No. 4,939,263, the best ratio obtained for xcex1,3,6-penta to xcex1,5,6-penta is 0.55, albeit at relatively low conversion.
Because of the difficulty in obtaining xcex1,3,6-penta by direct chlorination, clopyralid is often manufactured from 3,5,6-trichloro-2-trichloro-methylpyridine (xcex1,3,5,6-hexa) or from 3,4,5,6-tetrachloro-2-trichloromethyl-pyridine (xcex1,3,4,5,6-hepta) by hydrolysis to the corresponding 3,5,6-trichloro- or 3,4,5,6-tetrachloro-2-pyridinecarboxylic acids followed by selective reduction; see U.S. Pat. Nos. 3,971,799; 4,087,431; and 4,778,576.
It would be desirable to have a direct chlorination process with improved selectivity to xcex1,3,6-penta.
It has now been found that the ratio of xcex1,3,6-penta to xcex1,5,6-penta obtained by chlorination of xcex1,6-tet in the vapor phase can be increased, i.e., the selectivity to xcex1,3,6-penta can be increased, by conducting the chlorination in the presence of a Type L zeolite catalyst. The present invention concerns an improved process for chlorinating 6-chloro-2-trichloromethylpyridine (I) 
in the vapor phase at elevated temperatures to obtain a chlorination mixture containing 5,6-dichloro-2-trichloromethylpyridine (II) and 3,6dichloro-2-trichloromethylpyridine (III) 
wherin the improvement comprises contacting the 6-chloro-2-trichloromethyl-pyridine (I) with chlorine in the presence of a Type L zeolite catalyst to obtain a mixture enriched in 3,6dichloro-2-trichloromethylpyridine (III). The ratio of xcex1,3,6-penta to xcex1,5,6-penta is greater than 0.75, preferably greater than 1.0. Preferably a Type L zeolite in the K- or [K, Na]-form can be used. Alternatively, the Type L zeolite catalyst can be doped with a Lewis acid catalyst, preferably a zinc containing Lewis acid catalyst, most preferably zinc chloride. Also alternatively, the Type L zeolite can be partly ion exchanged to substitute other elements for potasium. Examples of suitable elements include, but are not limited to Group I and II metal cations.